When, for example, converting a color space of image data into a CMYK color space, expressing gray (gray color) by a combination of densities of four colors CMYK allows the gradation continuity between gray and a chromatic color to be maintained. On the other hand, expressing gray by a density of a single color K (black) allows gray to be simply and clearly expressed.
And also, an expressing method has also been well-known for confirming whether or not the image data includes a pixel showing a chromatic color, and if so, expressing all the gray colors in the image data by the combination of densities of the four colors CMYK, and if not, expressing the gray colors by densities of a single color K. This method provides unified gray colors in the image data all in a decided expression, when whether the gray is expressed by the combination of densities of the four colors CMYK or by a density of a single color K is decided.